The Author(s) 2018. This article is published with open access at www.chitkara.edu.in/publications.

ABSTRACT

Transition-metal doped Indium Phosphide (InP) has been studied over several decades for utilization in optoelectronics applications. Recently, interesting magnetic properties have been reported for metal clusters formed at different depths surrounded by a high quality InP lattice. In this work, we have reported accumulation of Ni atoms at various depths in InP via implantation of Ni- followed by H– and subsequently thermal annealing. Prior to the ion implantations, the ion implant depth profile was simulated using an ion-solid interaction code SDTrimSP, incorporating dynamic changes in the target matrix during ion implantation. Initially, 50 keV Ni- ions are implanted with a fluence of 2 × 1015 atoms cm-2, with a simulated peak deposition profile approximately 42 nm from the surface. 50 keV H- ions are then implanted with a fluence of 1.5 × 1016 atoms cm-2. The simulation result indicates that the H- creates damages with a peak defect center ∼400 nm below the sample surface. The sample has been annealed at 450°C in an Ar rich environment for approximately 1hr. During the annealing, H vacates the lattice, and the formed nano-cavities act as trapping sites and a gettering effect for Ni diffusion into the substrate. The distribution of Ni atoms in InP samples are estimated by utilizing Rutherford Backscattering Spectrometry and X-ray Photoelectron Spectroscopy based depth profiling while sputtering the sample with Ar-ion beams. In the sample annealed after H implantation, the Ni was found to migrate to deeper depths of 125 nm than the initial end of range of 70 nm.

INTRODUCTION

Diluted magnetic semiconductors (DMS), especially III-V semiconductors, have become a major field of research over the last few decades. As Si based technology has begun to approach its limit in production viability, size, and power, DMS materials have been receiving increasing attention as a possible alternative. InP based materials do not face the present issues found in Si based integrated circuits (ICs). Si ICs need low defect densities to control the carrier lifetimes and decrease the junction leakage current [1]. Whereas in DMS, the defect densities could be used to control the circuit current and can be utilized to enhance ferromagnetic properties at the same time [2]. Presently in the DMS materials, the ferromagnetic properties can only be observed below room temperature [2]. To utilize these materials practically, ferromagnetism must be exhibited with a Curie temperature (Tc), at or above the room temperature [3]. The DMS materials can be synthesized using several approaches ranging from epitaxial growth to ion implantation, all of which rely on producing a specific concentration of magnetic ions. To synthesize the DMS materials with existing methods, a 5% atomic concentration of transition metal dopant must be obtained [2]. This amount of doping causes numerous defects. These defects in the substrate lattice as well as the magnetic ions which have been incorporated into the lattice generate observable magnetic fields. These defects are believed to mediate the desired magnetic characteristics through the delocalization of hole concentration near the magnetic ion sites [3]. It is generally observed that annealing the samples at higher temperatures result in an increase of transition metal migration to cation lattice sites and hole density. A potential solution in synthesizing DMS materials which can exhibit ferromagnetic behavior at room temperature, is the creation of small metal precipitates within a semiconductor that independently exhibit magnetic functionality, rather than relying on defects proportional to the hole concentrations. These small metal precipitates become magnetic nanoclusters. They can be conceptualized as a bulk ferromagnetic material with a net magnetization and measurable hysteresis loop as a function of applied magnetic field for temperatures below Tc [4]. It has been theorized that synthesizing these magnetic nanoclusters in a high-quality crystal lattice will increase both their magnetic anisotropy and volume. These nanoclusters have been offered as an alternative to spin injection and collection in the semiconductor [5]. Irradiation with heavy ion beams along with post-thermal annealing has been an attractive technique for synthesis of metal clusters in semiconductors [6]. A major concern in this type of synthesis, is the ion induced damage in the substrate. The implanted region may be so heavily damaged by defects caused by the transition metals, that post implantation annealing may not fully restore a high quality crystal structure within the lattice [5]. This unfortunate defect mechanism associated with the implantation process can be overcome by a second ion implantation utilizing a light ion, such as H, subsequent to the initial implantation with a transition metal and high temperature annealing. This process can form nanoclusters and keep a high quality crystal phase in the semiconductor. The H implantation induces nano-cavities upon post implantation annealing to act as trapping sites for the transition metals by the dangling bonds left from the vacated H [7, 5, 8]. This works looks into using this novel technique to investigate re-distribution of Ni atoms at various depths beyond the initial heavy ion implant depth in the InP lattice. InP was specifically chosen for this research to look into the advancement of high power and frequency devices due to the high electron mobility inherent in this type of semiconductor.

In a single ion metal implantation into semiconductors, after annealing, the metal atoms typically migrate to either to the sample surface or to the end of the initial range involving the interface of amorphous and crystalline regions of the substrate. In the 50 keV Ni implantation into InP, the initial distribution of significant amount of Ni was up to a depth of 70 nm from the surface. However, after annealing the sample (which was initially Ni implanted along with further 50 keV H ion implantation), from the RBS analysis, the Ni was found at the top 20 nm and at another region with 25 nm width at a depth of 100 nm below the top layer. The initial Ni implantation fluence was kept at a lower level for complete recovery of the substrate crystalline quality. The XPS profiling was sensitive enough to detect the Ni concentration in the asimplanted samples. However, in the annealed samples, the Ni atoms have migrated deeper with a concentration level below the detection limit of the XPS profiling. This preliminary results have demonstrated that the implanted Ni atom have indeed migrated deeper (beyond the initial end of the range) into the bulk material, most likely accumulated in defect sites left by the vacated H. In the future to improve the elemental detection sensitivity, depth-profiling experiments are planned to utilize Secondary Ion Mass Spectroscopy (SIMS).